Fix #482: guard love.system.pickFile so Import ROM cannot crash

Pressing Import ROM on iOS takes the whole app down:

  src/import/RomImporter.lua: attempt to call field 'pickFile' (a nil value)

love.system.pickFile is a NATIVE BRIDGE, not part of LOVE. It exists only on
builds that compiled one -- Android, and iOS builds patched by
mobile/ios/patch_love_src.py -- so on a build without it the field is simply
nil. RomImporter:546 routes iOS down the same path as Android
(`mobileOS == "Android" or mobileOS == "iOS"`), and all three mobile pick
sites called the field unguarded.

That is why the reports say "any version": nothing about it is version
specific. Red, Blue and Yellow all reach the same call.

Every one of those call sites already handles a device with no document
picker -- Choose falls back to "No picker available, copy your ROM into:"
plus the save directory, and the mod / save rows have their own notices --
and love.system.createFile at its single call site was already guarded this
way. These three were not, so the fallback that was written for exactly this
case could never be reached.

Route them through one small helper that answers false when the bridge is
absent. A build without a picker now degrades to the copy-into-the-save-folder
flow, which on iOS is a working path: the Files app exposes the app's
Documents folder and GRBootstrap sweeps what lands there into the save dir.

tests/rom_importer_no_picker_test.lua covers Import ROM, Import mod and
Import save with the bridge missing, and asserts the picker is still used
when it is present. Reverting the fix reproduces the reported error exactly.

Reported in #482 (confirmed by three people) and #512.

src/import/RomImporter.lua

@@ -5,6 +5,25 @@ local HostShell = require("src.core.HostShell")
 local RomImporter = {}
 RomImporter.__index = RomImporter
 
+-- love.system.pickFile is a NATIVE BRIDGE, not part of LÖVE: it exists only on
+-- builds that compiled one (Android, and iOS builds patched by
+-- mobile/ios/patch_love_src.py). A build without it must fall back to the
+-- copy-it-into-the-save-folder flow that every caller below already has --
+-- calling the nil field instead took the whole app down the moment the player
+-- pressed Import ROM:
+--
+--   src/import/RomImporter.lua: attempt to call field 'pickFile' (a nil value)
+--
+-- love.system.createFile was already guarded this way at its one call site;
+-- these three were not. Every caller here treats `false` as "no picker
+-- available" and shows its own notice, so a missing bridge now degrades to
+-- exactly the path a picker-less Android device has always taken.
+local function pickFile(...)
+  local fn = love.system.pickFile
+  if not fn then return false end
+  return fn(...) and true or false
+end
+
 -- Cache generation tag; bump to force every imported version to re-extract.
 local CACHE_FORMAT = "rom-cache-v8:"
 -- The completion marker is written under each version's cache prefix
@@ -984,7 +1003,7 @@ function RomImporter:chooseMod()
         self.modNotice and self.modNotice.ok)
       return
     end
-    if not love.system.pickFile("mod") then
+    if not pickFile("mod") then
       self.modNotice = { ok = false,
         text = "Could not open the file picker. Copy a mod .zip via USB." }
     else
@@ -1047,7 +1066,7 @@ function RomImporter:chooseSaveImport(version)
       return
     end
     self.androidPendingVersion = version
-    if not love.system.pickFile("sav") then
+    if not pickFile("sav") then
       self.androidPendingVersion = nil
       self.saveNotice[version] = { ok = false,
         text = "Could not open the file picker. Copy a .sav via USB." }
@@ -1135,7 +1154,7 @@ function RomImporter:choose(version)
       self:startData(data, name)
     elseif consumePickedRomError(self) then
       return   -- a rejected pick explains itself instead of silently reopening
-    elseif not love.system.pickFile() then
+    elseif not pickFile() then
       -- Picker unavailable (API < 19, or no document-picker app installed):
       -- fall back to the USB folder-drop path as a friendly notice, not an
       -- error (which would read as a rejected file).